After a Malaysia Airlines flight was shot down over Ukraine, Transportation Secretary Anthony Foxx said Monday that intelligence and security teams constantly review threats to passenger planes.

The Federal Aviation Administration had issued a warning in April against flying over the Crimean Peninsula, south of where Malaysia Flight 17 went down Thursday, because of disputed air-traffic control in the region between Ukraine and Russia. The FAA expanded the warning Friday to all of eastern Ukraine.

"We are constantly in touch with our intelligence community, with our national security team, on these issues," Foxx said in response to questions after a speech at the National Press Club. "We continue to monitor the international situation to ensure that U.S. commercial carriers are given the best guidance possible."

Foxx noted that warnings for commercial flights are gleaned from trusted sources and that many countries have been covered worldwide.

Ukraine has been fighting pro-Russia separatists for months. Asked whether he regretted covering just Crimea in the initial warning, Foxx said the warning in April urged caution among U.S. airlines, and he said some avoided a broader area voluntarily.

"Our thoughts and prayers are with victims and their families," Foxx said of the Malaysia flight. "When the agency receives specific and credible actionable intelligence of a threat to the national airspace system – U.S. civil aviation and its passengers – the agency provides guidance to carriers and/or institutes flight restrictions."

•Ethiopia, where the FAA urges U.S. aircraft to avoid the northern region above 12 degrees latitude.

•Iran, where flights require advance planning for lack of U.S. diplomatic relations.

•Iraq, where flights below 20,000 feet are discouraged without special planning.

•Kenya, where "recent, credible information indicates a potential near-term terrorist attack against U.S. and Western interests." An Israeli flight from Mombasa was attacked in 2002 by portable anti-aircraft weapons.

•North Korea, which launched short- and medium-range missiles with no warning in February and March.